1. Field of the Invention
The present invention relates to the manufacture of semiconductor device, more particularly to a method of forming a bottle-shaped trench in a semiconductor substrate for formation of a capacitor for dynamic recess random memory (DRAM).
2. Description of the Related Art
As the integration density of a dynamic random access memory (DRAM) steadily increases, it becomes necessary to reduce the memory cell size. The memory cell size is primarily determined by the minimum resolution dimension of a lithographic technique, the overlay tolerances between the different features and the layout of these features. At the same time, it is necessary to maintain the minimum required storage capacitance to reliably operate the DRAM. To meet both the cell size requirement and the storage capacitance requirement, a trench capacitor was invented; the simple single device/capacitor memory cell has been altered to provide the capacitor in a vertical dimension. In such designs, the capacitor is formed in a trench in the surface of the semiconductor substrate. For example, U.S. Pat. No. 5,348,905 issued to Kenney on Sep. 20, 1994, entitled, xe2x80x9cMETHOD OF MAKING DIFFUSED BURIED PLATE TRENCH DRAM CELL ARRAYxe2x80x9d, teaches the basic elements and process steps for fabricating a buried plate DRAM cell structure.
However, as the size of a DRAM is scaled down by a factor of f (feature size), the trench storage node capacitance decreases by a factor of f. Therefore, it is important to develop methods to increase the storage capacitance. One method employed to increase capacitance is to widen the bottom portion of the trench, thus, increasing the surface area and creating a xe2x80x9cbottle shapedxe2x80x9d capacitor. However, in order to space the capacitors close together, control of the etching process used to widen the bottom portion becomes a governing factor. Chemical dry etching is predominantly used in the prior art for creating the bottle-shaped portion of the capacitor. In U.S. Pat. No. 5,112,771 issued to Ishii, et al. on May 12, 1992, entitled, xe2x80x9cMETHOD OF FABRICATING A SEMICONDUCTOR DEVICE HAVING A TRENCHxe2x80x9d, the bottom region of a trench capacitor is enlarged. This is accomplished by leaving a silicon oxide film on the upper sidewall of a trench, and enlarging the width of the exposed bottom portion of the trench by an isotropic dry etching process. Since the silicon substrate is isotropically dry etched, it is etched not only in the vertical direction to the surface of the substrate, but also in the horizontal direction. Although the capacitor surface area is enlarged, the etching processes are not easily controlled.
In view of the above disadvantages, an object of the invention is to provide a method of forming a bottle-shaped trench in a semiconductor substrate. According to the method, the capacitor surface area can be efficiently enlarged by a simpler way. That is to say, the formation condition of the bottle-shaped trench for capacitor can be easily controlled.
Accordingly, the above objects of are attained by providing a method of forming a bottle-shaped trench in a semiconductor substrate. First, a hard mask having an opening is formed on the semiconductor substrate. The semiconductor substrate is then etched through the opening of the hard mask to create a trench, by, for example, reactive ion etching, wherein the trench has a top portion and a bottom portion. Next, a nitride film is formed on the surfaces of the trench and the hard mask. Then, a shield material fills the bottom portion of the trench followed by removal a part of the nitride film to leave the nitride film protected by the shield material and expose the surface of the semiconductor substrate at the top portion of the trench. Next, the shield material is removed to expose the nitride film formed on the bottom portion of the trench. Afterward, an oxide film is formed on the top portion of the trench. Then, the nitride film is etched to expose the surface of the semiconductor substrate at the bottom portion of the trench. The semiconductor substrate is selectively etched through the bottom portion of the trench with a diluted ammonia solution as the etchant to form a bottle-shaped trench followed by removal of the oxide film. According to the method of the invention, the bottle shaped trench can be easily created by wet etch through the bottom of the trench. The end point of the wet etch can be easily determined by time mode.
Furthermore, the semiconductor substrate can be single-crystalline silicon substrate. Also, the hard mask preferably comprises a pad oxide formed on the semiconductor substrate and a silicon nitride formed on the pad oxide. The shield material can be formed by coating a photoresist material into the trench followed by partially removing the photoresist material formed at the top portion of the trench to leave the photoresist material formed at the bottom portion of the trench to serve as the shield material.
Furthermore, the nitride film is preferably silicon nitride formed by chemical vapor deposition using a silicon-containing gas and a nitrogen-containing gas as the reactive gas. The silicon-containing gas can be silane, dichlorosilane, or trichlorosilane, the nitrogen-containing gas can be ammonia or nitrogen gas.
Moreover, the oxide film is preferably silicon nitride formed by reaction of the silicon substrate in a high temperature ambient (650xc2x0 C.xcx9c800xc2x0 C.) containing oxygen and/or water.
Furthermore, the nitride film at the top portion of the trench can be removed by diluted phosphoric acid.
Furthermore, the above objects of the invention can be attained by providing a method of forming a bottle-shaped trench for capacitor in a semiconductor substrate. First, the semiconductor substrate is selectively etched to form a trench, wherein the trench has a top portion and a bottom portion. An oxide film is then formed on the top portion of the trench. Next, the semiconductor substrate is etched through the bottom portion of the trench by a diluted solution to form a bottle-shaped trench followed by removal of the oxide film.